For eight long months, Peta Murgatroyd let her size-seven dance shoes collect dust. Pregnant when she nabbed her second Dancing With the Stars trophy last May with model Nyle DiMarco, “I hadn’t done a step since,” the Aussie, 30, admits in the new issue of Us Weekly.

So after welcoming son Shai January 4, fiancé Maks Chmerkovskiy gave her a nudge, lining the basement of their New Jersey home with mirrors and enlisting instructors from his Dance With Me studios to serve as partners.

“I just pawned her off to them,” explains the season 18 champ, 37. “I was like, ‘I’m not going to be the guy telling her what to do after she just gave birth!’”

His bride-to-be (they’ll wed this July) made good use of the setup. Come March, the All Things Fam and Glam blogger will slip back into sequins. “Most people start using the stairs instead of the elevator at nine weeks postpartum — this one is going to be dancing full-out,” marvels Ukraine native Chmerkovskiy, who will join his fiancée on the ABC hit (premiering March 20 at 8 p.m.) Check out what they told Us in the video above. The full cast will be revealed on Good Morning America March 1.

Murgatroyd’s motivation is twofold. “I want to get my body back,” admits the 5-foot-7 pro. “And I want to defend my title. I won two seasons ago and never got to come back.” (Chmerkovskiy welcomes the challenge, telling Us, “I just want to support her, and at the same time, maybe have some healthy competition in the family.”)

And though dancing, running and weight training have helped Murgatroyd shed most of her baby weight (“I think I’m kind of halfway there,” she says), one thing does make her nervous about slipping back into her old skimpy costumes. “I’m not looking forward to having huge boobs on the show,” admits the breastfeeding star. “I’ve never had massive knockers before, and I don’t really know how we’ll tie them down! I actually contacted the costume designer and said, ‘We need to have a meeting about this!’”

For more on her and Chmerkovskiy’s return to the ballroom — and how they’re handling life as new parents — pick up the new issue of Us Weekly, on stands now!